Temporal Variation in Fixation Stiffness Affects Healing by Differential Cartilage Formation in a Rat Osteotomy Model

Dynamization involves a reduction in fixation construct stiffness during bone healing, allowing increased interfragmentary movement of the fracture through physiologic weightbearing and muscle contraction. Within some optimal range, interfragmentary movement stimulates healing, but this range likely varies across stages of bone healing. How does the time of dynamization affect the cartilage formation, bony bridging, and bone resorption in a rat fracture-healing model? Unilateral external fixators, stabilizing a 1-mm gap, were dynamized at 1 (D1 group, n = 10), 3 (D3 group, n = 11), or 4 (D4 group, n = 11) weeks postoperatively. Continuously 5 weeks stiff (S group, n = 10) and flexible (F group, n = 11) fixation were included for comparison. After 5 weeks, healing was evaluated by histomorphometric methods. Advanced healing, indicated by less cartilage and a greater rate of bony bridging, was observed in the S group compared to the D1 or F group. In contrast, the D3 and D4 groups had less cartilage and more bridging compared to the S group. Also, the S group had less cortical resorption than the F and D1 groups. These data suggest late dynamization at the onset of bony bridging led to enhanced healing, whereas dynamization at the early stage of cartilage differentiation delayed healing. Although our observations from this small-animal study cannot be directly transferred to humans, these data suggest, once bony bridging begins, dynamization may stimulate bone healing and accelerate remodeling.